Considerations for selecting and applying hot runners for molds

In order to exclude or reduce the failure in use as much as possible, the following matters should be noted when selecting and applying hot runner system.

1.The choice of heating method

Internal heating method: internal heating nozzle structure is more complex, the cost is higher, the parts are difficult to replace, the electric heating element requirements are higher. The heater is placed in the middle of the runner, will produce circular flow, increasing the frictional area of the capacitor, the pressure drop may be as much as three times the external heat nozzle.

But because the heating element of internal heating is located in the torpedo body inside the nozzle, all the heat is supplied to the material, so the heat loss is small and can save electricity. If a point gate is used, the tip of the torpedo body is kept in the center of the gate, which facilitates the cutting off of the gate after injection and makes the residual stress of the plastic part lower due to the late condensation of the gate.

External heating method: The external heating nozzle can eliminate the cold film and reduce the pressure loss. At the same time, due to its simple structure, easy processing, and thermocouple installed in the middle of the nozzle so that the temperature control is accurate and other advantages, currently in production has been commonly used. But the external heat nozzle heat loss is larger, not as energy-efficient as the internal heat nozzle.

2. The choice of gate form

The design and selection of the gate directly affects the quality of plastic parts. In the application of hot runner system, according to the resin fluidity, molding temperature and product quality requirements to choose the appropriate gate form, in order to prevent salivation, dripping material, leakage and color change bad phenomenon.

3.Temperature control method

When the gate form is determined, the control of melt temperature fluctuation will play a key role in the quality of plastic parts. Many times the scorched material, degradation or flow channel blockage phenomenon is mostly caused by improper temperature control, especially for heat-sensitive plastics, often require a rapid and accurate response to temperature fluctuations.

To this end, the heating element should be reasonably set to prevent local overheating, to ensure that the heating element and runner plate or nozzle with the gap to minimize heat loss, and should try to choose a more advanced electronic temperature controller to meet the temperature control requirements.

4.The temperature and pressure balance of the manifold calculation

The purpose of the hot runner system is to inject the hot plastic from the nozzle of the injection molding machine, pass through the hot runner at the same temperature and distribute the melt to each gate of the mold with a balanced pressure, so the temperature distribution of the heating area of each runner and the pressure of the melt flowing into each gate should be calculated.

Calculation of nozzle and gate sleeve center offset due to thermal expansion. In other words, it should be ensured that the center line of the hot (expanded) nozzle and the cold (not expanded) gate sleeve can be accurately positioned and aligned.

5.Calculation of heat loss

The internally heated runner is surrounded and supported by the cooled mold sleeve, so the heat loss due to heat radiation and direct contact (conduction) should be calculated as accurately as possible, otherwise the actual runner diameter will be smaller due to the thickening of the condensation layer on the runner wall.

6.Installation of runner plate

The two aspects of thermal insulation and injection pressure should be fully considered. Usually set up between the runner plate and the template cushion and support, which on the one hand can withstand the injection pressure, to avoid deformation of the runner plate and the phenomenon of material leakage, on the other hand, can also reduce heat loss.

7.Maintenance of hot runner system

For the hot runner mold, the use of regular preventive maintenance of hot runner components is very important, this work includes electrical testing, sealing components and connecting wire inspection and cleaning of components dirty work.

Post time: Jul-20-2022


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